autopep8 runthrough and janitorial changes.

In the interests of avoiding accidentally including janitorial changes in any possible future pull requests. I figured I'd might as well run waitress through autopep8. After reverting some of its more overly-enthusiastic changes and fixing a few other issues I noticed, here's the result.

In addition, this pull request includes the following fixes:

 * outbuf_overflow was documented with the incorrect default value.
 * In TestHTTPChannel, 'NotImplemented' was being raised as an exception instead of 'NotImplementedError', which appeared to be a typo.
 * A number of lists in waitress.utilities are now tuples.